1. Field of the Invention
This invention relates to the spray coating of annular workpieces.
2. Description of the Prior Art
In water pumps for automobiles, for example, and other machinery, small bushing-like parts comprising seals having a smooth and wear-resistant, annular sealing surface are used. Seals of ceramic material having smooth lapped faces have been used in the past for this duty. A sealing surface may also be provided by a hard coating applied to the metallic body of the seal.
When a coating is used to develop the sealing surface, it is important that the coating be applied with as smooth and uniform a surface as possible so that the dressing time and material removed in dressing is minimized.
In an attempt to accomplish this uniformity, coating material has been sprayed onto the surface to be coated. In the past, coating material has been applied by arranging a plurality of the pieces in close proximity to each other on a work surface with their annular faces to be coated facing away from the work surface, and by directing the spray gun at the work surface and moving the gun so that the centerline of the spray describes a series of parallel lines. By this method, only a fairly uniform and smooth coating of material is applied to the annular surfaces of the pieces. Since the spray patterns generally lay a path of material having feathered sides or edges, the spacing of the parallel passes is critical to the achievement of a coating thickness of overall uniformity. Consequently, more material than necessary is often applied and then removed during dressing of the surface in time-consuming, wasteful, and inefficient operations.
Also, because the pattern of the spray covers much more area than the annular surfaces to be coated, e.g., the entire work surface upon which the pieces are arranged, a great deal of coating material is sprayed between the annular surfaces and in the holes of the pieces contributing to further significant waste. In addition, since it takes several passes of the gun to coat each piece, this spraying method is slow and inefficient. SUMMARY OF THE INVENTION
The method of the present invention sharply reduce the waste of coating material which has resulted from previous spraying methods. Using the present invention, coating material is sprayed onto the annular surface only, not between the surfaces and not in the hole of the annular workpiece. As a result, the spraying of unnecessary areas is avoided and dramatic savings in coating material are obtained.
The present invention also provides a more efficient method of coating annular surfaces both in terms of time of application and of time removal and amount of material required to be removed in order to achieve the desired finished surface. By the method of this invention, the entire annular surface can be sprayed in a single pass beneath the spray gun. Thus a series of annular workpieces can be sprayed rapidly.
These advantages and others are accomplished by the method of the present invention, in which relative translational motion is produced between a plurality of workpieces having annular surfaces facing in a direction generally normal to the motion and a source of spray material such as a plasma spray gun directed at and spaced from them a relatively fixed distance. Preferably, the workpieces are passed successively beneath the spray gun as it simultaneously reciprocates toward and away from the path of travel of the workpieces. As each workpiece passes beneath the spray, the gun goes through one oscillation. The movement of the workpiece and the oscillation of the gun are synchronized so that the gun follows the curvature of the annular surface passing it. At the same time, the workpiece is rotated, bringing the entire annular surface within the field of the spray device to provide the surface with a smooth uniform layer of the coating material.
Alternatively, the method comprehends having the gun translated relative to a stationary array of workpieces. In both the case of a stationary gun and translated workpieces and the case of a translated gun and a stationary array of workpieces, the gun may either be oscillated toward and away from the path of relative translational motion or the workpieces may be moved in an oscillating motion or wavelike path toward and away from the path of relative translational motion.
A particular case comprehended by the method comprises arraying the annular workpiece faces to be coated in a staggered row, i.e., with their centers lying on an alternating wave path. When such an array is translated relative to a gun following a non-oscillating path the advantages and benefits of the method are also realized.
In all of the alternate forms of the method described above, the annular workpieces are rotated about their centers at least during the coating operation.
The preferred embodiment of the current invention uses a turntable to carry the workpieces past a spray gun and a camming mechanism to provide the reciprocating motion to the gun. A series of belt and chain drives synchronize the motion of the cam with the passage of the workpieces by the gun. One motor drives the cam and the turntable assembly. Another motor provides the rotating movement of the workpieces by means of an eccentric drive mechanism.